Virtualization promises increased utilization rates, better manageability of the IT infrastructure and cost reduction, but you can bet you'll spend a chunk of your expected savings on new hardware – including memory, CPUs designed for virtualization, or entirely new servers, users say.

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If you plan to run many virtual machines (VMs) on a single server, without question, you'll need to increase memory on the host system, said Joe Clabby, an analyst at Yarmouth, Maine-based Clabby Analytics.

For many users who anticipate saving money from a consolidation project, the unanticipated outcome of having to spend substantial money to meet virtualization requirements for additional RAM, CPUs or otherwise is a hard pill to swallow.

Virtualization brings new hardware requirements One storage architect who works for a major aerospace corporation and could spoke anonymously, said if you want to run 20 to 30 VMs per physical host, you'd better start shopping for extra RAM.

"Each of the hosts on our local ESX farm regularly handles 20 to 30 VMs. The hosts are eight-way Xeons [with two quad cores] with 64 GB of RAM each. Memory is your main problem if you're running that many VMs on a single box. Even if you limit each one to 512 MB of RAM, that's still [a ton] of RAM," the storage architect said.

"While it's easy to share CPU time and even disk space, each server basically needs its own amount of physical RAM," he said. "If each server needs 512 MB of RAM--which I'd say is the minimum for useful work on a Windows server—that's 2 servers per GB of RAM on the host."

VMware recommends a minimum of 512 MB RAM to run VMware ESX Server 2.0. Microsoft's soon to be released Hyper-V hypervisor requires 512 MB for the parent partition, plus the allocated memory another 32MB overhead for each "child partition." Therefore, a child partition that has 256 MB allocated virtual RAM requires a host that has at least (512 + [256+32] = 800 MB of RAM, reported independent technology analyst and author Brian Madden on his blog.

In addition, Hyper-V requires an x64-based processor, and hardware-assisted virtualization processors – so anyone who hasn't upgraded to these new processors can't run Hyper-V.

Over the past year, hardware-assisted virtualization processors have been introduced by IBM Corp., with its Power 6, Intel Corp. with Intel-VT and AMD with AMD-V to increase VM performance.

And if memory and CPU upgrades won't cut it, entirely new servers may be in order, Clabby said.

"If you have old [Intel] Xeon tower servers and want to consolidate and virtualize them, it makes sense to upgrade your servers to new architectures," Clabby said.

In general, Clabby said, "You absolutely do not have to buy new servers and dump all of your old stuff, but if you want the best performance when consolidating, then it is a good idea."

According to another user, another unanticipated pitfall is the need to spend the money you expected to save. "As the years go on, we have a running tab of money spent on beefier hardware versus costs we avoided with virtualization. Pretty quickly, the avoided costs outpaced the up-front costs to get virtualization hardware," said Erik McCloud, a Citrix local area network administrator with Trinity Health.

McCloud suggested budgeting for virtualization projects with the expectation that some of it – or all – will be redistributed to elsewhere.

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